The present invention relates to coupling circuits for coupling signals between components in electronic systems. Electronic system performance often falls short of expectations based on component/device specifications, so that system designers routinely derate devices they incorporate.
The operation of many electrical and electronic systems relies on the application of a voltage across a load from a voltage source and it is recognized in the art that the performance of such systems depends on the quality of the coupling between the source and the load, i.e. the extent to which the voltage across the load corresponds to, or equals, the open circuit source voltage or an amplified version of the source voltage.
Ideal coupling would result in the appearance, across the load, of a voltage, or possibly a current, exactly proportional to the open circuit source voltage with respect to both time and frequency domain characteristics as the source signal.
The achievement of ideal coupling would be advantageous in many electronics systems. For example, if the signal source is an electronic or electrical circuit port, the load is an oscilloscope employed to monitor the time domain waveform of the voltage developed by the signal source and the intervening coupling medium is the oscilloscope probe and associated cabling, the oscilloscope trace will accurately reflect the waveform of the source voltage only if the voltage applied to the oscilloscope inputs is identical to the open circuit source voltage.
For many other types of electronic systems, ideal coupling would produce optimum results. This would also be the case in a variety of audio, data transmission, communication and telecommunication systems. However, there is no known circuit arrangement which achieves perfect transfer, or ideal coupling, of the source voltage to the load.
Known coupling circuits fail to achieve ideal coupling for several reasons. Firstly, the voltage source is connected to the load via a coupling medium which has a finite impedance at any given frequency, and which will therefore be responsible for a voltage drop. Secondly, a practical voltage source behaves like an ideal voltage source in series with a source impedance. Therefore, when current is drawn from such a source, there is a voltage drop across the source resistance.
It is known to minimize source-to-load voltage drops, or signal attenuation, by the provision of an active circuit, known as an amplifier stage, between the source and load circuits. When such a circuit is used, for example when an operational amplifier is used as a unity gain voltage follower, the ratio of load voltage, V.sub.L, to source voltage, V.sub.i, can be expressed as follows: ##EQU1## Where A.sub.v is the open circuit voltage gain of the amplifier stage, R.sub.IN is the driving point input resistance of the amplifier stage,
R.sub.OUT is the driving point output resistance of the amplifier stage, PA1 Z.sub.i is the source impedance, and PA1 Z.sub.L is the load impedance.
If R.sub.IN is much greater than the absolute value of Z.sub.i, the absolute value of Z.sub.L is much greater than R.sub.OUT, and A.sub.v is approximately equal to 1, the load-to-source voltage ratio will be almost equal to unity. An ideal amplifier will have a value for R.sub.IN approaching infinity, a value for R.sub.OUT approaching zero and a value for A.sub.v substantially equal to unity.
However, practical amplifiers, when connected as a unity gain voltage follower, or amplifier, which include bipolar emitter followers and MOSFET source followers, have characteristics which are far from ideal. Emitter followers typically establish driving point input resistances that are rarely larger than a few hundred k.OMEGA. and driving point output resistances that are rarely smaller than several tens of .OMEGA.. Additionally, their open circuit voltage gains are usually no better than 0.95. MOSFET source followers provide a reasonable approximation of an infinitely large driving point input resistance, but their output resistance can be of the order of 100.OMEGA.. Moreover, the low frequency open circuit voltage gain of a MOSFET source follower can be as low as 0.75 and when compared with bipolar emitter followers, the frequency response of a MOSFET source follower is substantially inferior. Both of these types of followers have marginal high frequency response capabilities. In the case of an emitter follower circuit, the frequency response can also be significantly underdamped, which would promote circuit and system instability, particularly when the load is highly reactive.
When a signal source is coupled to a load having an impedance with a reactive component, the current through the load is out of phase with the voltage across the load. Conventional coupling devices, including conventional amplifiers and impedance buffers, can not supply the correct out-of-phase load current. As a result, when the load is reactive or has a reactive component, even the best coupling devices can not avoid distorting the signal across the load. Such distortion is particularly apparent during those portions of the signal cycle when the load current polarity is opposite to the load voltage polarity. Those in the art will recognize that the magnitude of the reactive component of a load impedance which will be undesirable is that which causes a load voltage distortion that measurably deteriorates the performance of the system in which the circuit is installed.
In addition, all coupling devices have some reactances, which may be undesired parasitic reactances, which will, in the prior art, unavoidably produce a time delay between the source voltage and the load voltage, as well as phase shifts within the coupling device which can lead to load voltage distortion.
Furthermore, in the operation of known coupling devices, input signal variations tend to induce power supply voltage variations. As a result, it has proven difficult to employ a single power supply to drive a plurality of coupling devices.
This is not because the devices fail to meet specifications, but because they are not provided with the required power supplies; lead lengths, ground loops, and noise pickup degrade the power delivered to the device.
A typical electronic signal processing system is shown in FIG. 1. The amplifier symbol represents any signal processor, such as an amplifier, that accepts an input signal v.sub.s and delivers a corresponding processed signal v.sub.o to a load impedance represented by a resistance. The ground symbol represents signal, or system, ground. The signal processor requires operating power from voltages V+ and V-, which come through "lines" from "power supplies" +E and -E with respect to ground. The power supplies are not necessarily of equal voltage, and often one of them is absent.
In common practice, the signal processor (hereafter "amplifier") is created by an integrated circuit "chip designer" and then a "system designer" incorporates the amplifier and other devices into a system that has to meet certain requirements. The interface between the chip designer and the system designer is a data sheet that contains the specifications of the amplifier, which always include the assumption of ideal power supplies to be provided by the system designer. That is, with reference to FIG. 1, E and therefore V+ and V- are assumed to be ideal zero-impedance voltage sources at all frequencies of interest, and the power supply lines are likewise assumed to be ideal noiseless zero-impedance connections.
When the system designer does indeed provide such ideal voltages V+ and V- at the amplifier, he expects the voltage v.sub.o delivered to his system load to be within the amplifier specifications. For example, a square wave input v.sub.s would deliver a rounded square wave v.sub.o according to the bandwidth specification of the amplifier, as in FIG. 1. As the amplifier "pulls" the load voltage v.sub.o above or below ground, the corresponding load current is drawn from the V+ and/or V- voltages and flows through the power supplies E as shown qualitatively in FIG. 2.
Such ideal performance, unfortunately, is rarely achieved: power supplies are nonideal, power supply lines are not zero-impedance, and noise (e.g. from "ground loops") can be injected into the power supply ground return. These nonidealities, and particularly the load current flowing in the nonzero line impedances, cause the amplifier output voltage to be distorted as indicated in FIG. 3, even when the signal processor is operating properly. It does not deliver specified performance simply because it is not being operated with the specified power supplies. This nonideality is often expressed in terms of "Power Supply Rejection Ratio", which ideally should be infinite, but in practice of course is not. In FIG. 3, as well as in FIGS. 4, 5 and 7, elements n represent noise sources and not actual signal sources.
This problem is well-known to system designers, whose usual "fix" is to place decoupling or "bypass" capacitors, as shown in FIG. 4, whose function is to filter the power supply voltage variations from V+ and V-, and to divert the load current from flowing in the nonzero line impedances, thus restoring essentially the specified amplifier performance.
Nevertheless, there are two (technical) limitations to this fix. First, the bypass capacitor, which is intended to be a zero impedance, is not ideal at both ends of the frequency spectrum: its finite capacitance causes it to be an infinite impedance at zero frequency, and it self-resonates at some high frequency at which it appears to be an "equivalent series resistance" (esr) and above which it behaves like an inductance (esl) causing it to be an infinite impedance also at infinite frequency. Indeed, often four capacitors are used instead of two to spread the desired low-impedance property over a wide enough frequency range.
Second, it is difficult to place the bypass capacitors physically close enough to the V+, V- terminals and system ground. This difficulty is greatly aggravated when, as often happens, system ground is some distance from the amplifier chip. As a result, the bypass capacitors actually perform as though they were tapped part way along the line impedances and noise source. Both of these technical limitations can be represented as in FIG. 5, from which it is obvious that the bypass capacitor fix can provide only limited improvement.